The present research program examines the developmental changes which occur in the capacity for acquiring language. More specifically, it examines what appears to be a sensitive period for the acquisition of the grammar of a language. The first part of this research program is descriptive. Four studied are proposed to investigate the structural parameters and reliability of this effect using a population of second language learners of varying ages. An additional study will evaluate the uniqueness of this effect for language by testing for age differences in music learning. The second part of this program is experimental. It will explore some of the changes that occur in the learning mechanism over development which may underlie and explain the descriptive phenomena. In particular it will explore possible child-adult differences in the use of the operating principles used to convert speech input into stored data. Seven studies are proposed. In each study, two groups of children of the ages of 6-7 and 10-11 years and one group of adults will be asked to learn a miniature artificial language. The miniature language will be designed to test the effects of morphological properties (including position, frequency, stress and structure) on the success and patterning of subjects' learning. In addition to documenting developmental changes in the use of these operating principles, some studies will test for the influence of first language typology on the use of such principles. Understanding age related changes in language learning capacity has important implications for educating the deaf (who are often not exposed to a native language at birth) and educating the many newly arriving child and adult immigrants to the United States. Such information will ultimately help us to develop educational opportunities tailored to the particular qualities inherent in different aged learners.